Target Strength(TS) is an important factor for the detection of target in active sonar system, so that the numerical model for the prediction of TS is widely being developed. For the frequency range of several kHz, the most important scattering mechanism is known to be the specular reflection, which is largely affected by the geometrical shape of the target. In this paper, a numerical algorithm to predict TS is developed based on Kirchhoff approximation which is computationally efficient. The developed algorithm is applied to the canonical targets of simple shape, for which the analytical solutions exist. The numerical results show good agreement with the analytical solutions. Also, the algorithm is applied to more complex scatterers, and compared with the experimental data obtained in the water tank experiment for the purpose of verifying the developed numerical model. Discussions on the effect of spatial sampling and the other aspects of the numerical modelling are presented.